1. Field of the Invention
The present invention relates to a three-layered catalyst system and, more particularly, to a three-layered catalyst system for purifying the exhaust gases discharged from internal combustion engines, in which a lower layer, including platinum or a platinum-rich precious metal component, an intermediate layer, including palladium or a palladium-rich precious metal component, and an upper layer, including platinum or a platinum-rich precious metal component, are sequentially layered on a substrate.
2. Description of the Related Art
Typical examples of exhaust gases discharged from transportation means, such as automobiles and the like, include hydrocarbons (HC), ozone (O3), particulate matter (PM), nitrogen oxides (NOx), carbon monoxide (CO), and sulfur dioxide (SO2). Since these harmful materials cause primary pollution, in which harmful gases are directly emitted in the atmosphere, and secondary pollution, in which exhaust gases chemically react with each other in the atmosphere and thus harmful gases are secondarily generated, the seriousness thereof is increasing day by day. The test modes for evaluating exhaust gases discharged from automobiles used worldwide are classified into American type test mode and European type test mode. Korea has applied the FTP-75 mode, which is the American type test mode, to automobiles, passenger cars, and small freight vehicles.
The FTP-75 mode is a system for evaluating the amount of constituents of exhaust gases discharged from automobiles when the automobiles are running in a predetermined running mode under regulated conditions, and includes a chassis dynamometer enabling the driving of automobile wheels, a sample collecting apparatus for collecting samples from exhaust gases discharged from test cars, and an exhaust gas analyzer for analyzing the collected samples. In this FTP mode, in the case of automobiles using gasoline or LPG as fuel, exhaust gases, such as HC, CO, NOx and the like, are analyzed, and, in the case of diesel automobiles, particulate matter as well as these exhaust gases can be evaluated.
The FTP-75 mode is performed through three steps. Here, a cold start phase, which is the first step, is performed during the period from 0 to 505 seconds, a cold start transient phase, which is the second step, is performed during the period from 505 to 1369 seconds, and a hot start phase, which is the third step, is performed for during the period from 0 to 505 seconds. According to the results of evaluation of the amount of harmful materials of exhaust gases depending on the types of cars, which were announced by the Government for the year 1998, it was reported that, in the case of automobiles using gasoline or LPG as fuel, the ratio of NOx is 15.5, PM 1.5, CO 52.1, and HC 56.6, in the case of buses, the ratio of NOx is 20.7, PM 24.4, CO 11.1, and HC 9.8, and, in the case of trucks, the ratio of NOx is 63.8, PM 74.1, CO 36.8, and HC 33.6. As noted in the report, in the case of the automobiles using gasoline or LPG as fuel, accounting for 68.8% of all types of vehicles, hydrocarbons (HC) are discharged the most.
Hydrocarbons cause primary pollution in that they are discharged in a state in which no fuel is combusted and secondary pollution in that they are mixed with nitrogen oxides (NOx), which are other harmful gases, and thus photochemical smog is produced by intensive sunlight.
Most of these hydrocarbons are discharged in a cold start phase during the period from 0 to 505 seconds after starting, that is, before a catalyst can operate. Therefore, hydrocarbons are first excessively discharged as soon as the engine is started.
As a method of evaluating a catalytic converter, which can reflect the characteristics of the cold start phase, a method of measuring light-off temperature (LOT), which is the temperature at which the catalyst reaches a predetermined activity, is used. However, in the method, the LOT is slowly increased, but the LOT is rapidly increased in real cars. Therefore, in order to make up for the discrepancy in the rate of LOT increase, a fast light-off test (FLOT) is used as a method of effectively evaluating the performance of a catalytic converter including a catalyst itself in consideration of rapid temperature increase at the time of engine starting. The fast light-off test (FLOT) is a method of accelerating the rapid temperature increase in real cars at the time of engine starting and measuring the performance of a catalytic converter, including a catalyst itself, depending on times.
The performance of a catalytic converter is represented by the time (time 50) at which the conversion ratio of HC, CO, and NOx is 50%. Therefore, the time at which the temperature of the catalytic converter reaches the temperature at which the catalytic converter is operable is a function of variables, such as the length of an exhaust pipe, the cell density of a support, the type of catalyst, the shape of the catalytic converter, and the like, and becomes an eigenvalue.
Examples of conventional automobile exhaust gas reducing apparatuses include a two-catalyst system and a purifying apparatus including a high heat-resistance catalyst. In the two-catalyst system, a catalyst is additionally mounted at a location adjacent to an engine, and the added catalyst is small in size and is located at the location adjacent to the engine, thus rapidly increasing purification efficiency immediately after the engine is started. Further, in the purifying apparatus including a high heat-resistance catalyst, since the high heat-resistance catalyst must be mounted at the location near the engine in order to increase purification efficiency rapidly, a thermally-durable catalyst, which can stand the high temperature of the engine, is used. As the catalyst, a precious metal selected from the group consisting of platinum (Pt), palladium (Pd) and rhodium (Rh) is used.
Based on the concept of the purifying apparatus, a conventional catalyst system for purifying exhaust gases has been advanced. Specifically, in the conventional catalyst system, a two-catalyst system is mounted on a ceramic substrate and a hydrocarbon adsorbing layer is additionally placed beneath the two-catalyst system. Referring to the operating principle of the convention catalyst system, first, the hydrocarbon adsorbing layer adsorbs hydrocarbons excessively discharged in a cold start phase, the adsorbed hydrocarbon is desorbed when the temperature at which a catalyst can be operated is reached, and then the desorbed hydrocarbons are removed by the catalyst.
However, a conventional catalyst composition for adsorbing hydrocarbons is disadvantageous in that the initial adsorption rate of hydrocarbons is low, and the hydrocarbons are desorbed before the time at which the catalyst can be operated is reached, thus decreasing the efficiency thereof (U.S. Pat. No. 6,074,973). Therefore, the catalyst composition for adsorbing hydrocarbons must have properties such that the efficiency of adsorbing excessively-discharged hydrocarbons must be high, and the desorption temperature of hydrocarbons must also be high, so as to maintain the hydrocarbons in an adsorbed state until the temperature at which the catalyst can be operated is reached.
In order to decrease the amount of hydrocarbons discharged from automobiles, various attempts have been made to maximally adsorb hydrocarbons discharged in a cold start phase and to develop a catalyst composition having a high desorption temperature. As an example of these attempts, U.S. Patent Application Publication No. 2001-0006934 discloses a catalytic converter having a multilayered catalyst system, in which a catalyst system and a hydrocarbon adsorbing layer are multi-layered on a ceramic substrate. However, the catalytic converter disclosed in this patent document has a problem in that, since hydrocarbons adsorbed at a low temperature are desorbed at an extremely low temperature, the hydrocarbons are discharged outside before they are oxidized at a high temperature.
Thus, the present inventors have made efforts to develop a highly efficient catalyst composition for adsorbing hydrocarbons. As a result, the present inventors produced a catalyst composition for adsorbing hydrocarbons including 1) a hydrocarbon adsorbing layer, formed of zeolite, applied on a substrate and 2) a two-catalyst system applied on the hydrocarbon adsorbing layer, in which the hydrocarbon adsorbing layer includes a water absorbing layer. The invention was registered as Korean Patent No. 496069.
Recently, due to efforts to improve an engine in order to decrease the amount of nitrogen oxides (NOx) included in exhaust gases discharged from automobiles, the temperature of exhaust gases has been gradually decreased, and the necessity for a catalyst composition having low activity at low temperatures has increased. FIG. 1 is a graph showing the exhaust temperatures of automobiles measured using an automobile meeting Euro-4 regulations and an automobile meeting Euro-3 regulations. In FIG. 1, it can be found that the exhaust temperatures of automobiles are gradually decreased as the regulations become strict. Here, the present inventors pay attention to the fact that the concentrations of HC and CO included in exhaust gases discharged from automobiles are increased depending on the decrease in the temperature of exhaust gases (shown in FIG. 2).